This invention relates to light weight porous structures for glass bodies such as a boule of high purity fused silica such as HPFS(copyright) fused silica glass or ULE(copyright) ultra low expansion glass of Corning Incorporated, Corning, N.Y. 14830. More specifically, the porous structures are supports for high purity fused silica mirror blanks.
Reducing the weight of mirror supports is vital for two applications. First, they are vital for the space program which has stringent weight constraints. Second, large scale terrestrial bound mirrors (over a meter in size) also desire light weight supports in order to have better control over the maneuverability of the mirrors while in use. The mirror supports should not add any stresses to the actual mirror. One of the best ways of doing this is to make the mirror support out of the same material as the mirrors so that no stresses from thermal expansion mismatch develop. Currently, mirrors are made of either high purity fused silica such as HPFS(copyright) fused silica glass or ULE(copyright) ultra low expansion glass of Corning Incorporated, Corning, N.Y. 14830. An additional requirement is that the mirror material must also be environmentally stable.
The existing method for making supports first involves making dense glass. Secondly, the glass is machined into an intricate honeycomb-like pattern to remove most excess weight. Thirdly, the mirror is bonded to the fabricated honeycomb structure. A process for making porous glass without first making dense glass would be highly desirable.
The processes in this invention describe methods, using flame hydrolysis and incomplete combustion for making porous light weight glass which can be used for mirror supports. The composition of the glass can be controlled. Even though high purity fused silica and ultra-low expansion compositions were the focus of the invention, the method is not restricted to these compositions. The process in this invention allows for the fabrication of controlled layered structures such that pore free glass can be made directly on top of the porous structure. The thickness of the layers and number of the layers are controlled in this process.
The existing manufacturing method of supports first involves making dense glass. Secondly, the glass is machined into honeycomb-like pattern to remove most excess weight. Thirdly, the mirror is bonded to the fabricated honeycomb structure. This invention describes a method of making structural supports for mirrors out of porous glass. The porous glass may still require machining, but machining of porous glass will be much less expensive than the standard dense glass. Bonding to the mirror blank should not change for the porous glass since thin layers of dense glass can be manufactured onto the porous glass.
Porous glass is made utilizing flame deposition of pure silica or doped silica in a manner similar to the production of high purity fused silica. Bubbles or seeds are formed in the glass during laydown. Several means of creating and controlling these seeds are available. Two approaches are demonstrated. The first approach involves the use of a liquid feed burner in which organic precursors such as liquid octamethyl cyclo tetra siloxane (OMCTS) or OMCTS and titanium IV tetra isopropoxide (tipox) droplets are injected into the cavity. Results suggest that unreacted OMCTS impacting the boule creates bubbles (seeds). The second approach for creating porous structures involves the use of conventional vapor fed bumers. Both of these processes use incomplete combustion to create bubbles. A number of different approaches can be used to create bubbles.